The invention relates to novel catalysts, electrode coatings and electrodes for the preparation of chlorine. Chlorine is usually produced industrially by electrolysis of sodium chloride or hydrochloric acid or by gas-phase oxidation of hydrogen chloride (Schmittinger, Chlorine, Wiley-VCH 1999, pages 19-27). If electrolysis processes are used, the chlorine is produced at the anode. As anode material, use is usually made of titanium as electrode material on the surface of which an electrochemically active catalyst is present. The surface layer containing the catalyst is usually also referred to as a coating. The tasks performed by the catalyst are reducing the overvoltage and avoiding evolution of oxygen at the anode (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, pages 469-470).
In the preparation of chlorine by electrolysis of hydrochloric acid, graphite anodes are usually used (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, page 514). In the electrolysis of hydrochloric acid, in which, for example, a gas diffusion electrode is used on the cathode side, it is possible to use titanium anodes which have coatings of noble metal-based catalysts (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, page 515).
Electrodes for electrolysis processes are usually based on a metal belonging to the group of “valve metals”. For the present purposes, valve metals are, for example, the metals titanium, zirconium, tungsten, tantalum and niobium. Owing to oxide layers on the metal surface, these act as diode material for electric current.
For use in electrolysis, an electrocatalytically active catalyst of a noble metal and/or its metal oxide is usually applied to the surface of the valve metals, with oxides of the valve metal optionally also being present in the metal oxide (WO 200602843 (ELTECH), BECK, Electrochimica Acta, Vol. 34, No. 6. pages 811-822, 1989). The oxide-forming noble metal is usually a platinum metal such as iridium, ruthenium, rhodium, palladium, platinum or mixtures thereof. Such electrodes are usually referred to as DSAs (“dimensionally stable anodes”).
Disadvantages of these known electrodes for use in halide-containing electrolytes are the still high overvoltage required for evolution of chlorine, the tendency of the electrodes nevertheless to evolve oxygen, the high electrolysis voltage and the high requirement of costly noble metal for producing coatings. All these factors have an adverse effect on the economics of the known electrolysis process using such electrodes.
It is also known (DE 602005002661 T2=US 2005/0186345) that the noble metals can be eluted from the coatings of the prior art over time under electrolysis conditions, and the coatings are accordingly not sufficiently corrosion-resistant over the long term. The necessity of corrosion resistance is made clear by the fact that the loss of noble metal-containing coating leads to the electrode metal, usually the valve metal, coming into direct contact with the electrolyte and forming an oxide which is not electrically conductive on its surface. For the on-going electrolysis process, this means that electrochemical processes no longer take place on this surface, which can result in total failure of the electrolysis cell with the corresponding adverse economic consequences.
Furthermore, when an electrolyzer having noble metal-containing DSAs is used in chloride-containing solutions for preparing chlorine, it has been observed that the secondary reaction of oxygen formation cannot be fully suppressed, as a result of which oxygen is found in the product chlorine gas. The proportion of oxygen results in an increased outlay for purifying the chlorine gas and thus likewise has adverse effects on the economics of the electrolysis. The increased formation of oxygen becomes clearly apparent particularly when the chloride concentration in the electrolyte drops, in the case of electrolysis of sodium chloride solutions especially at a concentration below 200 g/l of NaCl, and when the current density is increased, especially above a current density of 5 kA/m2.
Furthermore, the sole use of noble metals as catalytic electrode material likewise impairs the economics of known electrodes because of the high price and shrinking availability on the world market of these metals.
Various approaches to the production of composite electrodes in order to replace or reduce the proportion of noble metal are known from the literature.
There have thus been attempts to use carbon-containing coatings for electrodes in electrochemical processes. For example, diamond-containing coatings can be applied by CVD (chemical vapor deposition) processes to electrodes. In the case of electrolysis in a sodium sulphate anolyte containing sulphuric acid, the coating is not stable and flakes off. Furthermore, the coatings have defects and electrode metal was therefore exposed to electrochemical corrosive attack. (AiF research project 85 ZN, 2003 to 2005, final report for the period 1.01.2003 to 31.03.2005 “Entwicklung and Qualitätssicherung stabiler Diamant-beschichteter Elektroden für neuartige elektrochemische Prozesse”). The research project was terminated because the technical objective was not achieved.
Catalysts containing finely divided, carbon modifications on electrodes for preparing chlorine are known from DE102009035546A1=U.S. Pat. No. 8,492,303. A fundamental disadvantage of carbon-containing electrodes is that carbon is not thermodynamically stable in respect of oxidation. Thus, the formation of volatile carbon-oxygen species, in particular carbon dioxide, carbon monoxide, carbonic acid, hydrogencarbonate or carbonate, is basically possible under anodic electrolysis conditions in the presence of oxygen which is always obtained as by-product in any electrolysis carried out in an aqueous electrolyte. Furthermore, carbon-chlorine compounds such as carbon tetrachloride can also be formed in the anodic production of chlorine. The loss of the carbon component from composite electrodes considerably reduces the mechanical stability and the life of such electrodes. In the case of pure carbon electrodes, as were formerly used, for example, in the preparation of chlorine by electrolysis using the amalgam process, burning leads to an increased electrode spacing and increased energy consumption and likewise to a reduced life of the electrode.
Even when catalysts described in DE102009035546A1 were used at sodium chloride concentrations below 200 g/l, these carbon-containing electrodes are still unstable in the case of malfunctions (for example of the brine supply to the electrolysis cells) or incorrect operation (for example switching-on of a cell in the water-flushed state), which greatly restricts the practical usability compared to carbon-free dimensionally stable anodes (DSA) according to the prior art.
Although the use of nonaqueous electrolysis could prevent evolution of oxygen at carbon-containing electrodes as secondary reaction, it has other disadvantages, e.g. the instability of organic solvents in respect of chlorine or the low ion conductivity of such solutions, which ultimately leads to a higher energy consumption. The known use of other water-free systems such as salt melts requires a comparatively high operating temperature which can be realized only at great expense and has been found to be uneconomical compared to aqueous solvents. Furthermore, the stability of electrodes and cell materials at very high temperatures is still a challenge and limits the life of corresponding electrolysis plants.
Furthermore, the production of the carbon modifications mentioned in DE102009035546A1, e.g. diamond and fullerene, is technically difficult and associated with a high economic outlay, so that these materials are often not available in industrially required quantities and with a consistent quality.
Electrodes whose electrocatalytically active coating consists of a matrix of an electrically conductive material having electrocatalytic properties in which a nonconductive particulate or fibrous refractory material is embedded are known from DE2245709A1. The resistance to damage caused by a short circuit when used as anode in combination with a mercury cathode is improved thereby. As regards the power consumption or the overvoltage, the fibre-containing electrodes (see Ex. 1 and 3 in DE2245709A1) do not differ from fibre-free electrodes produced analogously or, when particulate materials are used (Ex. 2 in DE2245709A1) have a significantly increased overvoltage compared to Ex. 1. In terms of radiation protection, the thorium oxide mentioned in Example 2 is also undesirable in commercial electrolysis plants because of its radioactivity.
It was therefore an object of the invention to discover a catalyst by means of which the electrolysis of chloride-containing aqueous solutions can be carried out at a low electrolysis voltage even at a low chloride concentration and in which the proportion of noble metal is also reduced. A further object was to discover an electrocatalytic coating which adheres firmly to the substrate metal and is not attacked either chemically or electrochemically and has at least the same effectiveness as known coatings. A chemically stable and inexpensive catalyst which is low in noble metals should likewise be made available for the gas-phase oxidation of hydrogen chloride.